Bottlebrush Polymers for Functional Materials

Bottlebrush polymers, also known as molecular brushes, are macromolecules with polymeric side-chains (Figure 1). Due to their large size (single molecules can exceed lengths of 100 nm) and densely crowded side-chains, bottlebrush polymers are candidates for a number of potential applications, including rheological modifiers, single-molecular rheological tracers, polymeric photonics,and drug-delivery vehicles. However, the structural details of bottlebrush polymers in solution and in the bulk are poorly understood. For example, while the overall size of bottlebrush polymers has been measured, the polymer stiffness and side-chain conformation have not been quantified. Furthermore, the conformational properties of the polymeric side-chains, including their flexibility and degree to which they are stretched, have not been elucidated. Finally, lyotropic liquid crystal (LC) phases in concentrated and semi-dilute solutions of bottlebrush polymers have received little attention, even though a lyotropic LC phase was first observed over 15 years ago in bottlebrush polymers.

Figure 1. Schematic for a bottlebrush polymer. The schematic emphasizes the cylindrical shape of the molecules and shows that bottlebrush polymers can exceed lengths of 100 nm.

We are interested in developing new applications of bottlebrush polymers and elucidating the basic structure of bottlebrush polymers in solution and in thin films. Bottlebrush polymers may be particularly useful both for controlling surface properties and for encapsulation and release of foreign molecules. The first application is motivated by our hypothesis that the side-chains of bottlebrush polymers have significant conformational flexibility, and by tailoring their structure and length they can exhibit reversible chemical properties at the bottlebrush exterior. We are also interested in taking advantage of the large size of bottlebrush polymers as well as their structural versatility to develop molecules which can be used to encapsulate and deliver drugs to specific locations.

The images above show our synthetic approach and results from atomic force microscopy (middle right) and transmission electron microscopy (far right). AFM and TEM images show individual polymers in size ranging from 20 – 40 nm, and the conformation of the bottlebrush polymers can be rod-like or spherical depending on the side-chain length and grafting density.

Recently, (Publication 1 below) we studied bottlebrush polymer films comprised of bottlebrush polymers with hydrophilic and hyrophobic side-chains. Using contact angle measurements and X-ray photoelectron spectroscopy, we were able to detect changes to the film chemistry with exposure to different solvent. This work demonstrates the potential for using bottlebrush polymers or bottlebrush polymer additives to modify surface properties.Currently, we are carrying out studies to investigate the structure and conformation of bottlebrush polymers in solution. We synthesized selectively deuterated bottlebrush polymers for analysis by small-angle neutron scattering. These results provide a direct measure of the length and stiffness of bottlebrush polymers in solution